<p>This paper investigates the optimal integral sliding-mode control for a group of nonlinear connected vehicles, in which the directed communication topology and external disturbances are both taken into consideration. In order to find an optimal platoon control protocol, a novel control framework regarding both the integral sliding-mode and optimization is proposed for this platoon. In details, by developing the platoon tracking errors system with directed communication topology, an integral sliding-mode control strategy is presented such that the platoon tracking errors would converge the sliding surface in finite time. The system stability and string stability are both analyzed. Then, an objective function in term of the tracking errors and the control inputs are presented to enhance the system robustness. Based on the Hamiltonian theory, an optimal nominal control strategy inside the proposed control framework is presented, such that the vehicles would coverage to the desired platoon with optimal robustness. Comparing with the traditional integral sliding mode based platoon control methods, the proposed strategy can solve the tracking problems with only a directed communication topology and shows better robustness. In final, numerical simulations are presented to verify the effectiveness and performance of proposed approaches.</p>